The present invention relates generally to crystal growth and, in particular, to crystal growth using the Czochralski method.
Crystal pulling from a melt is at present the primary method for production of a variety of crystal materials of industrial importance with diameters of up to and in excess of 7 inches and lengths up to and in excess of 7 feet. A melt containing the desired proportions of materials is placed in an appropriate container and is heated under a gas atmosphere to above its melting point by means of a heat source which may be a resistance, radio frequency or electron beam heater. With the melt at the desired temperature, a crystal seed of specified crystallographic orientation is contacted with the melt surface from above and the temperature is adjusted such that crystal-melt contact is maintained and the materials of the melt solidify onto the seed material in a manner replicating the seed structure upon lifting of the seed crystal which is attached to a lifting device. During subsequent crystal pulling, the diameter of the growing crystal being pulled is increased or decreased as desired by appropriate adjustments of parameters such as the melt temperature. In order to accomplish growth of materials which exhibit an excessive vapor pressure at the freezing point, the melt may be encapsulated with an immiscible inert fluid (of lower density than the melt) such as, for example, B.sub.2 O.sub.3, and growth is conducted at ambient pressures in excess of the vapor pressure of the most volatile melt constituent.
It is a fact that even under optimized use of the controllable growth parameters, the conventional process of pulling crystals from a melt results in materials with crystalline and chemical perfection which is significantly less than theoretical. The material deficiencies can largely be attributed to lack of heat transfer control within the hot zone which gives rise to asymmetry in thermal field distribution within the melt and to uncontrolled and time, varying axial and radial temperature distribution in the growing region of the crystal.
More stringent property requirements in materials such as silicon and gallium arsenide, dictated by emerging device technology, have established a need for effective heat transfer control during crystal growth of single crystals. In particular there is need to control the axisymmetric thermal gradients about the growth interface and along the growing crystal. In particular the axisymmetric heat losses are primarily derived from convective heat transfer under turbulent conditions when growing GaAs, InP and CdTe.
For example, in U.S. Pat. No. 4,579,949 which is incorporated by reference, a heat pipe 12 is placed about a growing crystal 30 to provide a controlled radial thermal gradient. The heat pipe 12 is placed in close proximity and above the surface and about the crystal emerging from the melt. The heat pipe 12 is not in direct contact with the growing crystal or the melt. A heating and/or cooling means 26 is attached by insulating means 24 to the heat pipe 12 to provide greater control over the temperature environment at the crystal/melt interface 34. This is shown in contact with the melt. The position of the heat pipe 12 can be changed by the axial positioning means 28.
It is also noted in this patent that the heat pipe 12 is constructed of stainless steel, nickle-based alloys, molybdenum-base alloys or quartz.
Although this patent helps to minimize temperature variations, greater control of the environment is still desirable.